Join/see UV's of different objects
Just previewing other UVs
There is an option in the UV Editor which - under certain circumstances - allows to see the UVs of other objects. Those circumstances are:

both objects use the same material, the material is active, and the texture is assigned
before entering Edit Mode, both objects have been selected
the Draw other Objects option is checked in the Image Editor (see screenshot below)


Editing other UVs at the same time
Use the Texture Atlas Add-on shipped with Blender to do this. After enabling it in the Add-on manager, you'll find a panel in the Properties Editor of the Render Settings tab. Select all objects which should receive the same UV space first, and then hit the + key to add a new Atlas. Then, hit the Start Manual Unwrap button.

All objects will be joined, you can edit the UVs as if they were one. In fact, at the moment they actually are one object, so pay attention beforehand to your modifiers. When you're done, hit the Finish Manual Unwrap button to separate the objects back to their original state.